Conventionally, as a power unit of this kind, one disclosed e.g. in Patent Literature 1 is known. This power unit is for driving left and right drive wheels of a vehicle, and is provided with an internal combustion engine, and first and second generator-motors. The first generator-motor has a first stator formed by a plurality of armatures, an intermediate rotor formed by an iron core and a coil, and an inner rotor formed by a permanent magnet. The first stator, the intermediate rotor, and the inner rotor are arranged in this order from the inside to the outside in a radial direction. Further, in the first generator-motor, an induction machine is formed by the first stator and the intermediate rotor, and a synchronous machine is formed by the intermediate rotor and the inner rotor. The above-mentioned second generator-motor has a second stator formed by a plurality of armatures, and a second rotor formed by a permanent magnet.
The intermediate rotor of the first generator-motor is mechanically connected to the crankshaft of the engine, and the inner rotor of the same to the second rotor of the second generator-motor, and the second rotor is mechanically connected to the drive wheels. Further, the first rotor of the first generator-motor and the second stator of the second generator-motor are electrically connected to a battery via a first controller and a second controller, each formed by e.g. an inverter.
In the conventional power unit constructed as described above, during running of a vehicle on which the power unit is installed, the power of the engine is transmitted to the drive wheels, while changing the speed thereof in the following manner: When the rotational speed of the engine is higher than that of the drive wheels, part of the power from the engine is used to generate electric power by the first generator-motor. This converts the part of the power from the engine into DC power by the first stator, whereby power generation is performed, and at the same time, the remainder of the power from the engine is electromagnetically transmitted to the inner rotor via the intermediate rotor, and then is transmitted to the drive wheels. The electric power thus generated by the first stator is supplied to the second stator via the first and second controllers, and the power thus generated by the second rotor is transmitted to the drive wheels. From the above, the power of the engine is transmitted to the drive wheels while changing the speed thereof.
As described above, in the conventional power unit, transmission of power of the engine to the drive wheels is performed via a first path formed by the intermediate rotor, magnetism, and the inner rotor, and a second path formed by the intermediate rotor, magnetism, the first stator, the first and second controllers, the second stator, magnetism, and the second rotor. In the first path, the power of the engine is transmitted by a so-called magnetic path formed by magnetism generated in the intermediate rotor, and hence it is possible to obtain a relatively high transmission efficiency. On the other hand, in the second path, the power of the engine is transmitted via a so-called electrical path in which the power is once converted to DC power, and is then converted back into power, so that conversion loss of the inverter or loss by generation of Joule heat occurs to make the transmission efficiency via this path lower.
In the conventional power unit, due to the arrangement thereof, approximately half of the power of the engine is transmitted to the drive wheels by the electrical path via the second path, and hence the driving efficiency of the power unit becomes lower. Further, since the induction machine is formed by the first stator and the intermediate rotor, when the electric power is generated by the above-mentioned first stator, Joule heat is generated not only in the coil of the first stator but also in the coil of the intermediate rotor, which prevents the sufficient power generation efficiency from being obtained, to further lower the driving efficiency of the power unit.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a power plant which is capable of enhancing driving efficiency with which the power plant drives driven parts.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. 2000-197324.